10.9.2 Name lookup, templates, and accessing members of base classes

The C++ standard prescribes that all names that are not dependent on
template parameters are bound to their present definitions when parsing
a template function or class.1 Only names that are dependent are looked up at the point
of instantiation. For example, consider

Here, the names foo and N appear in a context that does
not depend on the type of T. The compiler will thus require that
they are defined in the context of use in the template, not only before
the point of instantiation, and will here use ::foo(double) and
A::N, respectively. In particular, it will convert the integer
value to a double when passing it to ::foo(double).

Conversely, bar and the call to foo in the fourth marked
line are used in contexts that do depend on the type of T, so
they are only looked up at the point of instantiation, and you can
provide declarations for them after declaring the template, but before
instantiating it. In particular, if you instantiate A::f<int>,
the last line will call an overloaded ::foo(int) if one was
provided, even if after the declaration of struct A.

This distinction between lookup of dependent and non-dependent names is
called two-stage (or dependent) name lookup. G++ implements it
since version 3.4.

Two-stage name lookup sometimes leads to situations with behavior
different from non-template codes. The most common is probably this:

In get_i(), i is not used in a dependent context, so the
compiler will look for a name declared at the enclosing namespace scope
(which is the global scope here). It will not look into the base class,
since that is dependent and you may declare specializations of
Base even after declaring Derived, so the compiler can't
really know what i would refer to. If there is no global
variable i, then you will get an error message.

In order to make it clear that you want the member of the base class,
you need to defer lookup until instantiation time, at which the base
class is known. For this, you need to access i in a dependent
context, by either using this->i (remember that this is of
type Derived<T>*, so is obviously dependent), or using
Base<T>::i. Alternatively, Base<T>::i might be brought
into scope by a using-declaration.

Another, similar example involves calling member functions of a base
class:

Again, the call to f() is not dependent on template arguments
(there are no arguments that depend on the type T, and it is also
not otherwise specified that the call should be in a dependent context).
Thus a global declaration of such a function must be available, since
the one in the base class is not visible until instantiation time. The
compiler will consequently produce the following error message:

x.cc: In member function `int Derived<T>::g()':
x.cc:6: error: there are no arguments to `f' that depend on a template
parameter, so a declaration of `f' must be available
x.cc:6: error: (if you use `-fpermissive', G++ will accept your code, but
allowing the use of an undeclared name is deprecated)

To make the code valid either use this->f(), or
Base<T>::f(). Using the -fpermissive flag will also let
the compiler accept the code, by marking all function calls for which no
declaration is visible at the time of definition of the template for
later lookup at instantiation time, as if it were a dependent call.
We do not recommend using -fpermissive to work around invalid
code, and it will also only catch cases where functions in base classes
are called, not where variables in base classes are used (as in the
example above).

Note that some compilers (including G++ versions prior to 3.4) get these
examples wrong and accept above code without an error. Those compilers
do not implement two-stage name lookup correctly.

Footnotes

[1] The C++ standard just uses the
term “dependent” for names that depend on the type or value of
template parameters. This shorter term will also be used in the rest of
this section.